Field of the Invention
The present invention relates to a solid-state image sensor and an image capturing apparatus.
Description of the Related Art
Recently, in an image capturing apparatus such as a digital camera, it is general that a CMOS image sensor is used for a solid-state image sensor, and image capturing for both a still image and a moving image is possible. While the number of pixels and image information amount of this CMOS image sensor are increasing, a user wants speedup in a readout operation which allows high-speed continuous shooting and a high frame rate in the moving image capturing.
In the moving image capturing, there are a live view mode in which the image is displayed on a liquid crystal screen and respective moving image formats such as HD, 4K, and 2K. When a pixel signal is read out as image information, the number of pixels needs to be reduced to the number corresponding to each format. At this time, if a method by thinning processing of reducing the number of pixels by skipping pixels in a certain cycle is used, it leads to deterioration in image quality such as the occurrence of moire. To cope with this, Japanese Patent No. 5250474 discloses a technique of performing pixel output mixture processing (averaging) in an image sensor and reading out the pixels at a high speed as a method of reading out the reduced number of pixels as a necessary information amount without losing the resolution of the image.
Japanese Patent No. 5250474 adopts an arrangement in which when mixing and then reading out pixel outputs, a plurality of rows whose pixel outputs are mixed are selected simultaneously, pixel signals on the same column are output on the same vertical signal line simultaneously, and the signals are mixed on the vertical signal line, thereby reading out the signals as the signals obtained by mixing the pixel outputs. This makes it possible, when reading out the mixed pixel outputs, to read out the pixel outputs at a high speed without providing a capacitor or the like separately.
In Japanese Patent No. 5250474, however, since there is only one vertical signal line for the pixel signals on the same column, mixture of the pixel outputs with respect to the same column is performed in the one vertical signal line. That is, it is necessary to sequentially read out the pixel signals on the same column from the one vertical signal line, and the pixel signals on one row need to be read out by temporary holding the readout pixel signals in a holding capacitor provided for each column and then performing horizontal transferring of the pixel signals sequentially. For this reason, the next row cannot be read out until a readout operation for one row is completed, thus taking a considerable time for the readout operation.